Apparatus for preventing the contamination of casting roll and the bulging of strip in twin roll strip caster

ABSTRACT

An apparatus for preventing bulging of both edges of a strip while preventing contamination of a roll surface in a twin roll strip caster including a meniscus shield ( 5 ) and a plurality of weirs ( 12 ). The apparatus comprises: first chambers ( 60 ) arranged at both sides of the meniscus shield ( 5 ) in a longitudinal direction parallel to the casting roll ( 1 ) and having inlet and outlet ports for non-oxidizing gas; second chambers ( 80 ) each assembled to an underside in each of the first chambers ( 60 ) in a communicating fashion for receiving non-oxidizing gas from the first chambers ( 60 ), and including a plurality of holes ( 81 ) formed in an inclined face thereof corresponding to an outer peripheral face in each of the casting rolls ( 1  and  1   a ) in a longitudinal direction of the each casting roll ( 1  or  1   a ); and passages S formed between the meniscus shield ( 5 ) and the second chambers ( 80 ) and reaching the gas outlet ports of the first chambers for allowing contaminated gas containing evaporated metal components and non-oxidizing gas injected from the second chambers ( 80 ) to be outwardly exhausted.

TECHNICAL FIELD

[0001] The present invention relates to an apparatus for preventingbulging of both edges of a strip while preventing contamination of aroll surface in a twin roll strip caster. More particularly, theapparatus of the invention mutually adjusts gas pressure in a firstspace between weirs for preventing molten level fluctuation and gaspressure in second spaces outside the weirs where rolls contact withmolten steel so that metal components evaporated from a surface ofmolten steel may not stick to roll edges, and outwardly exhausts themetal components evaporated in the second spaces mainly through acentral portion rather than through edge portions in respect to a rolllateral direction so as to realize a defectless strip through uniformcondensation in the roll lateral direction.

BACKGROUND ART

[0002] As shown in FIG. 1, in a conventional twin roll caster 100 feedsmolten steel from a turn dish 2 through an immersion nozzle 3 into aspace between dams 4 closely contacted to bath sides of casting rolls 1and 1 a so that a molten iron pool 13 is formed by the casting rolls 1and 1 a and the edge dams 4. A meniscus shield 5 is installed over thecasting rolls 1 and 1 a to prevent oxidation of molten steel owing tooxygen contained in air contacting with a surface of the molten ironpool 13, non-oxidizing gas is fed into a hollow space between anunderside of the meniscus shield 5 and the surface of the molten steelpool 13. As a result, the surface of the molten steel pool 13 contactswith non-oxidizing gas thereby restraining oxidation of molten metal bythe maximum amount.

[0003] A Japanese Laid-Open Patent Application Serial No. H6-297111proposes a sealing apparatus, which is arranged over molten steel leveladjust the depth of the immersion nozzle 3 immersed into molten steelwhile shielding the molten steel pool 13 surrounded by the casting rolls1 and 1 a and the edge dams 4 from an external oxidizing atmosphere inorder to prevent oxidation of molten steel in the molten steel pool 13.

[0004] According to the above document, where it is necessary to varymolten steel level during casting, adjustment of a spring mounted on alateral portion of the turn dish can regulate the immersed depth of theimmersion nozzle 3 while successively maintaining the atmosphere overthe molten steel surface.

[0005] Furthermore, a Japanese Laid-Open Patent Application Serial No.H7-204795 is aimed to install side dams or weirs which are partiallyimmersed into the molten steel pool 13 so as to prevent oxide created inthe molten steel surface from flowing into a solidification cell mixedwith molten steel. That is, this technology sets a certain gap betweeneach of the casting rolls 1 and 1 a and each of the side dams andmaintains the molten steel level within this gap and a space over moltensteel in a non-oxidizing gas atmosphere so as to prevent creation ofoxide by the maximum amount as well as created oxide from flowing intothe growing solidification cell mixed with molten steel.

[0006] However, even though the non-oxidizing atmosphere gas is filledover the molten steel surface, volatile components such as Mn, Zn and Pbescapes from the molten steel pool 13 into the atmosphere gas so as tomix with pure non-oxidizing gas thereby creating contaminated gas.

[0007] Since contaminated gas contains the above-mentioned volatilecomponents, it is condensed in contact with cold surfaces of the castingrolls 1 and 1 a and thus sticks thereto to influence heat transmissionof the casting rolls 1 and 1 a thereby deteriorating the quality of astrip 10. In order to avoid this problem, it is necessary to avoid thesurfaces of the casting rolls 1 and 1 a from contacting withcontaminated gas if possible.

[0008] Furthermore, since this technology has no means for controllingthe flow of volatile metal gas created from the molten steel surface,such volatile metal gas or contaminated gas contaminates the surfaces ofthe casting rolls 1 and 1 a thereby causing surface defects of the strip10 as well as degrading productivity.

[0009] Gas pipes 6 are installed at both sides of the meniscus shield 5parallel to the longitudinal direction of the rolls and connected to agas feeding line 9 for feeding non-oxidizing gas to block external gasfrom flowing into a lower space of the meniscus shield 5. A second flowof non-oxidizing gas such as nitrogen is also injected toward outerperipheral faces of the rolls to prevent external air from flowing intothe lower space of the meniscus shield as well as assist theabove-mentioned flow of non-oxidizing gas.

[0010] Molten steel dispensed via a nozzle hole 14 in a lower end of theimmersion nozzle 3 may create molten level fluctuation since it has avery large amount of momentum. In order to regulate molten steelfluctuation in the molten steel pool P, weirs 12 are installed along alongitudinal direction of the rolls with their upper ends fixed to weirsupports 17.

[0011] Although it is most preferred to maintain the space over thesurface of the molten steel pool 13 the non-oxidizing atmosphere, thespace is rarely-provided with perfect sealing and oxide is partiallyproduced in the molten steel level. The weirs 12 act as barriers toprevent above-produced oxide from reaching the growing solidificationcell.

[0012] In the molten steel pool 13 formed between the casting rolls 1and 1 a, specific substances such as Mn continuously evaporate from thesurface of molten steel in the lower space of the meniscus shield 5, andthe above-mentioned metal components or volatile impurities mix withnon-oxidizing gas fed onto the molten steel surface moving along thenon-oxidizing gas flow.

[0013] Since the volatilized substances such as Mn have a very low valueof thermal conductivity, they may function as a thermal resistance whendeposited on the surfaces of the casting rolls 1 and 1 a in formation ofa solidified cell 11, thereby creating regional bulging owing tonon-solidification and resultant defects of the strip 10.

[0014] Natural exhaustion is generally made in the space under themeniscus shield 5. However, when contaminated gas is produced exceedinga reference quantity, it is necessary to actuate a gas exhaust hose 8via an exhaust pump 7 to adjust the quantity of gas which is exhaustedto the outside.

[0015] As shown in FIG. 2, the flow of non-oxidizing gas is produced viaa gap between the edge dam 4 and the weir 12. When the gap between theweir 12 and the edge dam 4 is removed to clear the flow of non-oxidizinggas, that is, the weir 12 closely contacts with the edge dam 4, skull iscreated in a contacting region between the edge dam 4 and the weir 12 sothat the weir 12 may be damaged in some hostile situations therebysuspending casting.

[0016] Further, the edge dam 4 is sometimes vibrated in order torestrain skull creation on a surface of the edge dam 4. However, if theweir 12 is closely contacted with the edge dam 4, vibration acting onthe edge dam 4 may damage the weir 12. Accordingly, it is not preferredto closely contact the edge dam 4 with the weir 12.

[0017] As set forth above, non-oxidizing gas externally supplied intothe meniscus shield 5 is contaminated through mixture with theevaporated metal components from the molten steel pool 13 while flowingthrough the meniscus shield 5. When mixed gas or contaminated gascontacts with the surfaces of the casting rolls 1 and 1 a, theevaporated metal components are condensed into solid again to stick tothe surfaces of the casting rolls 1 and 1 a.

[0018] The evaporated solid components stuck to the casting rolls 1 and1 a obstruct heat transmission of the casting rolls 1 and 1 a and thusvary the thickness of the adjacent solidification cell 11, therebycausing cracks in solidification of a cast strip. As a result, it isnecessary to manage the casting rolls 1 and 1 a in such a manner thatcontaminated gas does not stick to the surfaces of the casting rolls 1and 1 a.

[0019] The present invention has been-made to solve the foregoingproblems of the prior art and it is therefore an object of the presentinvention to provide an apparatus for preventing bulging of both edgesof a strip while preventing contamination of a roll surface in a twinroll strip caster, which prevents contaminated gas formed throughmixture of non-oxidizing gas and the evaporated metal components fromcontacting and sticking to the roll surfaces or mixing into moltensteel.

DISCLOSURE OF THE INVENTION

[0020] According to an aspect of the invention for realizing the aboveobjects, it is provided an apparatus for preventing contamination ofroll surface and strip edge bulging in a twin roll strip caster whichincludes a meniscus shield 5 for interrupting inflow of external airwhile feeding non-oxidizing gas to prevent oxidation of molten steel ina molten steel pool 13 defined by casting rolls 1 and 1 a and edge dams4 and a plurality of weirs 12 mounted under the meniscus shield 5 forpreventing mold level fluctuation, the apparatus comprising: firstchambers 60 arranged at both sides of the meniscus shield 5 in alongitudinal direction parallel to the casting roll 1 and having inletand outlet ports for non-oxidizing gas; second chambers 80 eachassembled to an underside in each of the first chambers 60 in acommunicating fashion for receiving non-oxidizing gas from the firstchambers 60, and including a plurality of holes 81 formed in an inclinedface thereof corresponding to an outer peripheral face in each of thecasting rolls 1 and 1 a in a longitudinal direction of the each castingroll 1 or 1 a; and passages S formed between the meniscus shield 5 andthe second chambers 80 and reaching the gas outlet ports of the firstchambers for allowing contaminated gas containing evaporated metalcomponents and non-oxidizing gas injected from the second chambers 80 tobe outwardly exhausted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a perspective view illustrating a conventional twin rollstrip caster;

[0022]FIG. 2 is a perspective view illustrating mounted positions of anedge dam and a weir in the twin roll strip caster in FIG. 1;

[0023]FIG. 3 is a sectional view illustrating an apparatus forpreventing surface contamination of casting rolls and bulging of a stripin a twin roll strip caster of the invention;

[0024]FIG. 4 is a partial sectional view illustrating an apparatus forpreventing surface contamination of casting rolls and bulging of a stripin a twin roll strip caster of the invention;

[0025]FIG. 5 is a detailed view illustrating a second chamber adopted inan apparatus for preventing surface contamination of casting rolls andbulging of a strip in a twin roll strip caster of the invention;

[0026]FIG. 6 is a conceptual view illustrating an apparatus forpreventing surface contamination of casting rolls and bulging of a stripin a twin roll strip caster of the invention in use;

[0027]FIG. 7 is a plan view of a meniscus adopted in an apparatus forpreventing surface contamination of casting rolls and bulging of a stripin a twin roll strip caster of the invention; and

[0028]FIG. 8 is a perspective view illustrating first and second areasformed in an apparatus for preventing surface contamination of castingrolls and bulging of a strip in a twin roll strip caster of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] The following detailed description will present a preferredembodiment of the invention in reference to the accompanying drawings.

[0030]FIG. 3 is a sectional view illustrating an apparatus forpreventing surface contamination of casting rolls and bulging of a stripin a twin roll strip caster of the invention, FIG. 4 is a partialsectional view illustrating an apparatus for preventing surfacecontamination of casting rolls and bulging of a strip in a twin rollstrip caster of the invention, FIG. 5 is a detailed view illustrating asecond chamber adopted in an apparatus for preventing surfacecontamination of casting rolls and bulging of a strip in a twin rollstrip caster of the invention, FIG. 6 is a conceptual view illustratingan apparatus for preventing surface contamination of casting rolls andbulging of a strip in a twin roll strip caster of the invention in use,FIG. 7 is a plan view of a meniscus adopted in an apparatus forpreventing surface contamination of casting rolls and bulging of a stripin a twin roll strip caster of the invention, and FIG. 8 is aperspective view illustrating first and second areas formed in anapparatus for preventing surface contamination of casting rolls andbulging of a strip in a twin roll strip caster of the invention.

[0031] As shown in FIGS. 3 to 8, the apparatus 1 of the invention isinstalled in a longitudinal direction of casting rolls 1 and 1 a at bothsides of a meniscus shield 5 which covers an upper portion of a moltensteel pool 13. The apparatus 1 of the invention comprises first chambers60 and second chambers 80, and regulates the flow of non-oxidizing gasfed to outer peripheral faces of the casting rolls 1 and 1 a so thatcontaminated gas created through mixture between metal componentsevaporated from molten steel and non-oxidizing gas does not contact withthe casting rolls 1 and 1 a so as to obtain uniform solidification of astrip.

[0032] That is, the first chambers 60 are arranged at both sides of themeniscus shield 5 in a longitudinal direction parallel to the castingrolls 1 and 1 a for receiving non-oxidizing gas fed from an external gaspump. Each of the first chambers 60 has a plurality of plates assembledvia bolts to define a box-shaped internal space allowing contaminatedgas from the molten steel pool 13 to escape outside. In an upperportion, the each first chamber 60 has upper gas inlet ports 61connected to a gas feed line 61 a for feeding non-oxidizing gas andupper gas outlet ports 62 connected to a gas exhaust line 62 a forexhausting contaminated gas. The upper gas inlet and outlet ports 61 and62 are individually perforated in the upper portion of the each firstchamber 60, separated via a plurality of partitions. The each firstchamber 60 has lower gas outlet ports 64 perforated in an undersidethereof corresponding to the upper gas inlet ports 61, and lower gasoutlet ports 65 perforated in a lateral portion thereof corresponding tothe upper gas outlet ports 62. The lateral portion is opposed to each ofthe sides of the meniscus shield 5.

[0033] The gas feed line 61 a extended from the upper gas inlet ports 61of the each first chamber 60 is connected to the gas pump for feedingnon-oxidizing gas and having a manometer capable of measuring feedingpressure and flow rate. The gas exhaust line 62 a extended from theupper gas outlet ports 62 is communicated with an exhaust pump 7. Theupper gas outlet ports 62 are preferably provided with filter members 66so as to clear foreign substances from contaminated gas outwardlyexhausted from the each first chamber 60.

[0034] Each of the second chambers 80 is detachably assembled to theunderside of the each first chamber 60 and communicates with the lowergas outlet ports 64 formed in the underside of the each first chamber 60so as to receive non-oxidizing gas from the each first chamber 60. Theeach second chamber 80 includes a horizontal plate 80 a, a verticalplate 80 b, an inclined plate 80 c and end plates 80 d which areassembled to both ends of the plates 80 a, 80 b and 80 c via a pluralityof bolts to define a triangular internal space. The each second chamber80 has an inclined face corresponding to an outer peripheral face ineach of the casting rolls 1 and 1 a having a plurality of holes 81formed in a longitudinal direction of the each casting roll 1 or 1 a sothat non-oxidizing gas internally fed via the lower gas outlet ports 64can be injected to the outer peripheral face of the each casting roll 1or 1 a.

[0035] A plurality of gas inlet ports 82 are perforated in an upper faceof the each second chamber 80 to communicate with the lower gas outletports 64 of the each first chamber 60. Two partitions 83 a and 83 b areinstalled in the internal space of the each second chamber 80 in orderto divide flow of non-oxidizing gas into a central partial flow andlateral partial flows when non-oxidizing gas is injected toward theouter peripheral face of the each casting roll 1 or 1 a via the holes81.

[0036] Preferably, the inclined face of the each second chamber havingthe holes 81 is uniformly spaced from the outer peripheral face of theeach casting roll 1 or 1 a with a curvature substantially identical withthe same in order to readily control gas flow.

[0037] Passages S are formed between the second chambers 80 and the bothsides of the meniscus shield 5 so as to exhaust contaminated gas via thelower gas outlet ports 65 in the lateral portions of the first chambers,in which contaminated gas is formed through mixture of the metalcomponents evaporated from molten steel and non-oxidizing gas injectedfrom the holes 81 of the second chambers 80 toward the outer peripheralfaces of the casting rolls 1 and 1 a.

[0038] Preferably, the passages S are defined by the spaces betweenweirs 12 and the second chambers 80, vent holes 85 perforated in fixingplates 89 for fixing the second chambers 80 and the gas exhaust linesfor interconnecting the lower gas outlet ports 65 of the first chambers60.

[0039] Sealing members 24 block clearances between the first chambers 60and gas knives 6. Curtains 23 are installed in portions of the gasknives 6 to block clearances between lower ends of the gas knives 6 andthe casting rolls 1 and 1 a so as to prevent inflow of external air. Asa result, the casting rolls 1 and 1 a are rotated during casting toblock inflow of external air to a surface of the molten steel pool 13while outwardly exhausting contaminated gas via the upper gas outletports 62 of the first chambers 60.

[0040] The sealing members 24 are generally made of wool, and thecurtains 23 are made of steel foil.

[0041] Hereinafter it will be described about the operation and effectof the invention having the above construction.

[0042] Molten steel is supplied into the turn dish 2 via the immersionnozzle 3 to form the molten steel pool 13 between the casting rolls 1and 1 a and the edge dams 4, and the casting rolls 1 and 1 a are rotatedin opposite directions. Molten steel contacting with the casting rolls 1and 1 a is deprived of heat in directions toward centers of the rolls 1and 1 a to form a solidification cell 11 on surfaces of the rolls 1 and1 a, and slips through a roll nip to form a strip 10.

[0043] Non-oxidizing gas is fed into the internal spaces of the firstchambers 60 arranged at both sides of the meniscus shield 5 by the feedpump (not shown) which is connected to the gas inlet ports 61 in theupper portion of the first chambers 60 via the gas feed lines 61, and atthe same time, non-oxidizing gas is fed via the gas feed lines 6 a intothe gas knives 6 arranged outside the first chambers 60 so that thespace above the molten steel pool 13 maintains a non-oxidizingatmosphere during casting.

[0044] In sequence, non-oxidizing gas in the first chambers 60 is fedinto the second chambers 80 which have the gas inlet ports 82 in theupper portions communicating with the lower gas outlet ports 64 formedin the undersides of the first chambers 60. Although the internal spacesof the second chambers 80 are respectively divided into three parts,i.e., a central area and lateral areas in the longitudinal direction ofthe rolls via the partitions 83 a and 83 b, non-oxidizing gas isuniformly fed to the areas under the same pressure.

[0045] Non-oxidizing gas in the second chambers 80 is injected towardthe outer peripheral faces of the casting rolls 1 and 1 a via the holes81 which are formed in the inclined lower faces of the second chambers80, and joins with non-oxidizing gas from the gas knives 6 to formgas-flows reaching the molten steel pool 13 along the outer peripheralfaces of the casting rolls 1 and 1 a.

[0046] The metal components such as Mn are successively evaporated fromthe surface of the molten steel pool 13 into the space under themeniscus shield 10, and a portion of the evaporated components partiallymixes with non-oxidizing gas flowing along the outer peripheral faces ofthe casting rolls 1 and 1 a to form contaminated gas. Contaminated gasis produced in and then exhausted from first areas A between outer facesof the weirs 12 and interfaces of the casting rolls 1 and 1 a contactingwith molten steel, in which the weirs 12 have upper ends detachablymounted on supports 17 via bolts and lower ends immersed into the moltensteel pool 13.

[0047] A remaining portion of the metal components is created in asecond area B between the opposed weirs 12, and exhausted to the outsidevia exhaust ports 5 a of the meniscus shield 5 corresponding to thesecond area B.

[0048] Since contaminated gas created in the first areas A is upwardlyexhausted via the vent holes 85, which are formed in the fixing plates89 of the supports 17 for fixing the weirs 12 and introduced to thelower gas outlet ports 65 of the first chambers 60 along the passages Sbetween the meniscus shield 5 and the second chambers 80, it does notstick to the surfaces of the casting rolls 1 and 1 a.

[0049] In succession, since the lower gas outlet ports 65 are separatedfrom the upper gas inlet ports 61 via the partitions 63, contaminatedgas introduced into the lower gas outlet ports 65 is outwardly exhaustedvia the exhaust pump 7 communicating with the gas outlet lines 62 awithout mixing with clean non-oxidizing gas fed into the upper gas inletports 61.

[0050] Although exhaustion of contaminated gas as above is carried outbasically in a natural fashion, when contaminated gas is created bylarge quantities, it is also possible to regulate the amount of gasexhausted via the gas exhaustion lines 62 a by adjusting the suckingforce of the exhaust pump 7.

[0051] The first areas A between outer faces of the weirs 12 and theboundary surfaces of molten steel has a pressure larger than that of thesecond area B between the opposite weirs 12 so as to preventcontaminated gas from sticking to the outer peripheral faces of thecasting rolls 1 and 1 a. The pressure difference between the first areasA and the second area B is preferably set to about 100 mmH₂O or less.

[0052] Preferably, in the first areas A, the gas pressure is formedhigher in both lateral edge portions than central portions so as toprevent bulging at both edges of the strip.

[0053] The above apparatus 1 is applied to a strip caster 100 to caststainless steel, in which 304 stainless steel is typically in the formof an alloy containing 18 wt % Cr-8 wt % Ni together with about 1 wt %Mn. Mn has a melting temperature of about 1244° C., and evaporates inresponse to temperature drop to mix with non-oxidizing gas therebyforming contaminated gas. Herein, 100% nitrogen is used as non-oxidizinggas injected into the meniscus shield 5. Of course, other mixed-typenon-oxidizing gas can be used also.

[0054] When non-oxidizing gas, in particular, nitrogen gas is injectedinto the meniscus shield 5, a large quantity of evaporated metalcomponents, i.e., Mn gas was produced from the surface of the moltensteel pool 13. Evaporated metal components are generally absorbed to thesurfaces of the casting rolls 1 and 1 a, when they are not outwardlyexhausted. Thus, casting time is prolonged to influence the qualities ofthe strip thereby interrupting solidification. However, when the firstand second chambers 60 and 80 of the inventive apparatus 1 were appliedto both edges of the meniscus shield 5 together with the gas knives 6,it is observed that the quantity of mill scale produced on the surfacesof the casting rolls 1 and 1 a was remarkably reduced to 20% or less ofthat observed in the prior art.

[0055] Contaminated gas containing the evaporated metal components isexhausted from the first and second areas A and B. The varying pressurein the areas A and B were measured by using a pressure gauge such as amanometer to calculate the pressure difference between the first andsecond areas A and B. An experiment was made to observe influences ofthe pressure difference to bulging of the edges of the strip 10, andresults thereof are reported in Table 1. TABLE 1 Pressure Differencebetween First and Second Areas (ΔP) (100 mmH₂O) Edge Conditions of Strip(10) 0 Excellent 50 Good 100 Average 150 Hot Band 200 Hot Band andBulging

[0056] Regarding the experiment results, it will be understood that thepressure difference between the first and second areas A and B should bemaintained at 1100 mmH₂O or less so that the strip 10 can achieveexcellent edge conditions.

INDUSTRIAL APPLICABILITY

[0057] As set forth above, the present invention adjusts the quantityand pressure of non-oxidizing gas, which is fed into the space under themeniscus shield covering the surface of the molten steel pool betweenthe casting rolls and the edge dams and exhausted therefrom, to protectthe casting roll surfaces from sticking of contaminated gas containingmixture of the metal components evaporated from the molten level duringcasting in the internal atmosphere and non-oxidizing gas therebyreducing the thickness of mill scale on the casting roll surfaces to 20%or less of that in the prior art. As a result, this substantiallyprevents cracks owing to solidification nonuniformity of the stripsurface in the prior art. Furthermore, the invention eliminatesnon-solidification or bulging of the edges in the strip therebyimproving qualities of the strip by large degree.

[0058] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions canbe made without departing from the scope and spirit of the invention asdisclosed in the accompanying claims.

1. An apparatus for preventing contamination of roll surface and stripedge bulging in a twin roll strip caster which includes a meniscusshield for interrupting inflow of external air while feedingnon-oxidizing gas to prevent oxidation of molten steel in a molten steelpool defined by casting rolls, edge dams 4 and a plurality of weirsmounted under the meniscus shield for preventing mold level fluctuation,the apparatus comprising: first chambers arranged at both sides of themeniscus shield in a longitudinal direction parallel to the casting roll4 and having inlet and outlet ports for non-oxidizing gas; secondchambers each assembled to an underside in each of the first chambers ina communicating fashion for receiving non-oxidizing gas from the firstchambers, and including a plurality of holes formed in an inclined faceof each of said second chambers, said inclined faces having a shapecorresponding to an outer peripheral face in each of the casting rollsin a longitudinal direction of the each casting roll; and passagesformed between the meniscus shield and the second chambers and reachingthe gas outlet ports of the first chambers for allowing contaminated gascontaining evaporated metal components and non-oxidizing gas injectedfrom the second chambers to be outwardly exhausted.
 2. An apparatus forpreventing contamination of roll surface and strip edge bulging in atwin roll strip caster in accordance with claim 1, wherein the gas inletand outlet ports include: upper gas inlet ports perforated in upperfaces of the first chambers and connected to a gas feed line for feedingnon-oxidizing gas; upper gas outlet ports perforated in upper faces ofthe first chambers connected to a gas exhaust line, wherein the uppergas inlet and outlet ports are separated via a plurality of partitions;lower gas outlet ports in the undersides of the first chamberscorresponding to the upper gas inlet ports; and lower gas outlet portsin lateral portions of the first chambers corresponding to both lateralportions of the meniscus shield4.
 3. An apparatus for preventingcontamination of roll surface and strip edge bulging in a twin rollstrip caster in accordance with claim 2, further comprising filtermembers in the upper gas outlet ports for filtering foreign materials incontaminated gas which is exhausted via the upper gas outlet ports. 4.An apparatus for preventing contamination of roll surface and strip edgebulging in a twin roll strip caster in accordance with claim 2, whereineach of the second chambers includes: a plurality of gas inlet portsperforated in an upper portion thereof corresponding to the lower gasoutlet ports in each of the first chambers; and two partitions in acentral space thereof for dividing flow of non-oxidizing gas, which isinjected toward the outer peripheral face of the each casting roll viathe holes in the inclined faces of the second chambers, into a centralpartial flow and two lateral partial flows.
 5. An apparatus forpreventing contamination of roll surface and strip edge bulging in atwin roll strip caster in accordance with claim 1, wherein the inclinedface in each of the second chambers having the holes is uniformly spacedfrom the outer peripheral face in the each casting roll with asubstantially identical curvature for readily controlling gas flow. 6.An apparatus for preventing contamination of roll surface and strip edgebulging in a twin roll strip caster in accordance with claim 2, whereinsaid passages include spaces between the weirs and the second chambers,vent holes perforated in fixing plates for fixing the second chambersand an exhaust line for interconnecting the lower gas outlet ports ofthe first chambers.
 7. An apparatus for preventing contamination of rollsurface and strip edge bulging in a twin roll strip caster in accordancewith claim 1, wherein the weirs define first areas between outer facesthereof and molten steel interfaces which have a gas pressure largerthan that of a second area between the opposed weirs.
 8. An apparatusfor preventing contamination of roll surface and strip edge bulging in atwin roll strip caster in accordance with claim 7, wherein the firstareas and the second area maintain a pressure difference of about 100 mmH₂O or less.
 9. An apparatus for preventing contamination of rollsurface and strip edge bulging in a twin roll strip caster in accordancewith claim 7, wherein the gas pressure of the first areas is higher inlateral edge portions than in central portions to prevent bulging inboth edges of a strip.